Self-stimulation signal detection in an optical transmission system

ABSTRACT

In an apparatus and method for troubleshooting a transmission system comprising optical line amplifiers (OA), optical reflections can be detected irrespective if a data signal is present or absent. Each OA is equipped with a self-stimulation signal detection unit for generating a low frequency local code unique to the transmission system and dithering the outgoing signal in a controlled manner with the local code. Each OA attempts to detect its local code in the incoming signal by comparing the energy of the transmitted and received dithers. The presence of the local code in the incoming signal initiates alarms which unequivocally identify the faulted OA. Each OA selects its local code out of a bank of local codes, according to a priority scheme and re-selects its local code in case of conflicts.

This application is a continuation of application Ser. No. 08/588,176,filed Jan. 18, 1996 (pending).

FILED OF THE INVENTION

This invention relates to optical transmission systems and in particularto the provision of distinguishing a reflected amplified spontaneousemission condition (ASE) or oscillation from normal operation of theoptical transmission system.

BACKGROUND OF THE INVENTION

In an optical transmission system a loss of input signal alarm is animportant tool for determining that an optical cable has been broken,removed, or introduces a high loss.

Improvements in optical transmission methods have vastly enhanced theuse of the optical communication systems by increasing both the datarates and the distance over which optical signals are transmitted.Erbium doped optical amplifier (EDFA), one of the latest components inphotonic systems, replaces the regenerator (repeater) in manyapplications. An optical amplifier can amplify optical signals withoutoptically demultiplexing them, thereby avoiding the costs of multipleoptical receivers, multiple regeneration circuits and multiple opticaltransmitters. One of the major advantages of optical amplifiers is thatthey amplify whatever bit rate comes down the fiber. Even if thetransmission rate is boosted, the device will not need to be replaced.

On the other hand, there are no error counts possible between opticalamplifiers and therefore isolating the cause of a degraded error rate isnot a simple task. Consequently, there is a need to provide a method andapparatus for troubleshooting a chain of optical line amplifiers, whereno parity error counts are available.

In optical amplified systems, the reflection of a significant portion ofthe light leaving via a given fiber may cause problems with detection ofthe loss of the input signal on that fiber. If the reflected outgoinglight could be distinguished from the desired input signal, thenappropriate alarms or control actions could be initiated. The outgoinglight, that is then reflected, could be amplified signal and amplifiedspontaneous emission (ASE), as in the case of a bi-directional system,or could be just ASE, as in the case of a unidirectional system. Or, theoutgoing light could be a combination of signals and ASE from bothdirections in the case where there are more complex optical pathreflections.

Especially in bi-directional optical amplifier applications, reflectionscan cause an optical amplifier to oscillate despite the opticalisolators that may be present. This oscillation path can involve morethan one optical amplifier in the system and be quite complex.

Measurement of the strength of reflections is presently done with anoptical time domain reflectometer (OTDR) that sends strong short pulsesof light down a fiber and measures the signal returned. This is anaccurate method, but the OTDR is a relatively large and expensive pieceof test equipment that can not easily be used while there is traffic onthe fiber.

Optical frequency domain reflectometry may also be used to detect faultsin an optical link. According to this method, the optical frequency isvaried and optically coherent detection is used (IEEE PhotonicsTechnology Letters, Vol. 2, No. 12, pp. 902-904, December 1990), or anoptical source is modulated with a constant amplitude tone that is sweptin frequency (Applied Optics, vol. 20 no 10, pp. 1840-1844, 1981).

Another method for detecting a fault in an optically amplified system isto use a correlation of a specifically generated pseudo-random pulsesequence for reducing peak power requirements (Applied Optics, Vol. 22,No. 23, pp. 3680-3681, 1983).

Still another prior art method is to measure the amount of DC lightreflected back via a four port coupler. However, this method does notstimulate or consider the AC portion of the signals. The DC reflectionis used to determine if a large reflection from a broken fiber or openconnector is present so as to shut-down the output of the opticalamplifier for safety. In addition, this has been known to falselytrigger from low level reflection due to Raleigh scattering in thefiber. This method cannot be used in bidirectional systems.

There is a need to provide a means for detecting errors in atransmission system irrespective if a data signal is present or absent.There is also a need to distinguish optical reflections from validinputs when isolating a cable break in an optically amplified system.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide methods andapparatus for monitoring performance of optical transmission systemswhich minimize or overcome some or all of the above problems.

It is another object of the present invention to provide a system and amethod for distinguishing reflections or oscillations of an opticallyamplified transmission system from normal operation of the system.

Still another object of the present invention is to provide aninexpensive means for detecting reflections or oscillations in anoptically amplified system which operates with or without a data signalbeing present. Distinguishing optical reflections from valid inputs isimportant when isolating, for example, cable breaks in an opticallyamplified system.

Accordingly, the invention is directed to an apparatus for detecting afault at an optical amplifier module of a transmission system,comprising, means for modulating a laser of the optical amplifier modulewith a local dither, means for measuring an outgoing optical signal ofthe optical amplifier module to obtain a transmitted dither, means formeasuring an incoming optical signal of the optical amplifier module fordetecting a received dither, and means for processing the receiveddither and the transmitted dither for determining the ratio of theenergy in the received dither to the energy in the transmitted dither,indicating if the local dither is present in the incoming opticalsignal.

The invention is further directed to an optical transmission system witha plurality (N) of optical amplifiers, an apparatus for detecting afault at each amplifier A_(i) (iε 1,N!) comprising, means for modulatinga laser of the optical amplifier A_(i) with a local dither D_(ij) uniqueto the transmission system, means for measuring an outgoing opticalsignal to provide a respective transmitted dither D_(oi), means formeasuring a respective incoming optical signal for detecting arespective received dither D_(ii), and means for processing therespective received dither D_(li) and the respective transmitted ditherD_(Oi) for determining the ratio of the energy in the respectivereceived dither to the energy in the respective transmitted dither todetect if any local dither D_(fj) is present in the received ditherD_(lj), where f designates the amplifier A_(i) and any other amplifierconnected in the transmission system downstream from the amplifierA_(i).

The invention further includes a method for monitoring the performanceof an optical amplifier module of a transmission system comprising thesteps of, generating at the optical amplifier module a local dither,modulating the bias current of an optical source of the opticalamplifier module with the local dither, measuring an outgoing opticalsignal of the amplifier module to provide a transmitted dither,measuring an incoming optical signal of the amplifier module fordetecting a received dither, and determining the ratio of energy in thereceived dither to the energy in the transmitted dither fortracking-down the local dither in the incoming optical signal.

There is further provided a method for monitoring the performance of aWDM optical transmission system comprising the steps of, at each opticalamplifier A_(i) (iε 1, N!) of the transmission system, generating alocal dither D_(ij) (i ε 1, N!, (iε 1, M!) of a known energy, foridentification of the optical amplifier and of a transmission channel,modulating the bias current of a laser of the transmission channel withthe local dither to mix an outgoing signal with the local dither,measuring an incoming optical signal for detecting a received ditherD_(ii), measuring the outgoing optical signal for detecting atransmitted dither D_(oj), calculating the ratio of energy in thereceived local dither to the energy in the transmitted dither, anddetermining if a local dither D_(fj) is present in the incoming opticalsignal, where f designates the amplifier A_(i) and any amplifierconnected in the transmission system downstream from the amplifierA_(i).

Advantageously, the present invention provides a method and apparatuswhich is an inexpensive addition to an optical amplifier module andgives a good accuracy in identifying the magnitude of opticalreflections. Being built into the equipment, it does not significantlydisturb the traffic, and can be continuously or remotely monitored.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a transmission system with a plurality ofbi-directional line amplifiers equipped with self-stimulation signaldetection units of the invention;

FIG. 2 shows the connection of a self-stimulation signal detection unitwith the power monitoring blocks of a unidirectional optical amplifier;

FIG. 3 shows the block diagram of a self-stimulation signal detectionunit;

FIG. 4 shows a flow-chart for a method of selecting and assigning alocal code to an optical amplifier;

FIG. 5 illustrates a flowchart of a method for assigning priorities;

FIG. 6 illustrates the block diagram of a self-stimulation detectionunit according to another embodiment; and

FIG. 7 is a flowchart for the method for generating a local codeaccording to the embodiment of FIG. 6.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

This invention uses a low frequency modulation (dither) of the opticalpower being transmitted by an optical amplifier as a test signal.Performance of transmission systems using optical amplifiers must bemonitored so that faults in the operation of the system can be isolatedto faulty EFDAs or terminals, and maintenance personnel can bedispatched to the appropriate location with pertinent information andequipment to correct that fault.

The device and method according to this invention are provided fordetecting reflections by selecting a local code, modulating the laser ofthe respective amplifier with the local code, and attempting to detectthe local code on the amplifier input(s). The dither is thus used as atest signal for monitoring the operation of the respective amplifier,irrespective if information signals are present or not. To this end, theoptical signal is tapped at the optical amplifier for measuring theinput and output power of the optical signal. The measurement is thenused for adjusting the output power by controlling the laser current, orfor declaring loss of signal and initiating the corresponding alarms.

The method and apparatus of the invention use the power monitors thatare generally present in optical amplifiers for monitoring the receivedand transmitted signals to determine the energy of the transmittedsignal. Provisions are made for measuring the ratio of the energy in thedither present in the incoming optical signal to the dither energy inthe transmitted dither. This ratio is also that of the reflected signalpower to the transmitted signal power.

FIG. 1 illustrates a transmission system with a plurality ofbi-directional line amplifiers equipped with power monitor andself-stimulation signal detection units of the invention.

Four bi-directional line amplifiers 10, 20, 30 and 40 are connected onan optical fiber link between the terminal 60 (a transmitter/receiver)and terminal 70 (a receiver/transmitter). In optical systems withbi-directional amplifiers as illustrated in FIG. 1, signals are presentin different optical wavelength in both directions, namely from thetransmitter of terminal 60 to the receiver of terminal 70 and from thetransmitter of terminal 70 to the receiver of terminal 60. A similarblock diagram is applicable for unidirectional amplifiers, where onlyone incoming optical signal and one outgoing optical signal are to beconsidered.

Each amplifier is equipped with a power monitoring and self-stimulationsignal detection unit (PM & SSD) 110, 120, 130 or 140, respectively.Each unit 110, 120, 130 and 140, generates a local code C_(ij), which isunique to the respective amplifier. Index "i" defines the amplifier andis an integer taking values between 1 and N, where N is the total numberof amplifiers in the optical transmission link. N=4 in the embodimentillustrated in FIG. 1. Index "j" defines the code, and is an integertaking values between 1 and M, where M is the total number of codesavailable for the optical transmission link. M=6 in the embodiment ofFIG. 1. It will be explained later why the number M of codes availableis greater that the number of amplifiers N.

A low frequency dither signal D_(ij) is formed at each amplifier usingthe local codes C_(ij) which encode the intensity of the light of thelaser pump with a known modulation depth. The codes are selected so thatthey do not interfere with the codes transmitted by the neighboringamplifiers and are thus separately measurable. For example, the codesmay be selected to be phase independent orthogonal to each other. Inthis way, the step of detecting the input dither D_(Ii) and the outputdither D_(Oi) at each optical amplifier A_(i) comprises decoding themodulation depth of the dither signal in the incoming and outgoingsignals, respectively.

PM & SSD unit 110 directly modulates the laser of amplifier 10 so thatoutgoing signals 2' and 3 are modulated with a dither D_(1j), where jε1,M!. Similarly, outgoing signals 3' and 4 of amplifier 20 are modulatedwith dither D_(2j) generated by unit 120, where D_(2j) ≠D_(1j) ;outgoing signals 4' and 5 of amplifier 30 are modulated with ditherD_(3j) generated with unit 130, where D_(3j) ≠D_(1j) and D_(3j) ≠D_(2j); and outgoing signals 5' and 6 of amplifier 40 are modulated withdither D_(4j) generated by unit 140, where D_(4j) ≠D_(1j), D_(4j)≠D_(2j), and D_(4j) ≠D_(1j). Alternatively, it is possible to modulateother lasers present in the amplifier.

At amplifier 10, a fraction of each of the incoming signals 2 and 3' forthe respective direction of transmission is diverted to PM & SSD unit110 as input signals 11 and 13, respectively, while a fraction ofoutgoing signals 2' and 3 is diverted as output signals 12 and 14. PM &SSD unit 110 measures the dither signal D_(I1) that may be present inthe optical incoming signals 2 and 3' for the respective direction oftransmission, to detect the local code C_(ij). Dither signal D_(I1) maycomprise dithers from the amplifiers connected upstream from theamplifier under consideration. In this example, no dither should bedetected by unit 110 in the incoming optical signal 2 transmitted fromterminal 60 to terminal 70, while the dithers transmitted by amplifiers20, 30 and 40 may be detected in the incoming optical signal 3'traveling from terminal 70 to terminal 60. If a reflection has occurredin the direction 60 to 70, unit 110 will detect the locally generatedcode C_(1j) in the incoming signal 11. The ratio of the energy in thedither D_(I1) detected in the incoming signal and the energy in thetransmitted dither D_(1j) gives the value of reflection.

Similarly, a fraction of each of the incoming optical signals 3 and 4'is diverted at amplifier 20 as input signals 15 and 17, and applied tounit 120. A fraction of the outgoing optical signals 3' and 4 is alsodiverted as output signals 16 and 18, and input to PM & SSD unit 120.Unit 120 measures the dither D_(I2) present in the incoming signals anddetermines if its own code C_(2j) is present. The ratio of the energy inthe received dither D_(I2) and the energy in the transmitted ditherD_(2j) gives the value of reflection.

The majority of optical amplifiers currently in use are equipped withpower monitors. A brief description of some of the functions of thepower monitors which are of interest to this invention will be given inconnection to FIG. 2.

FIG. 2 shows the block diagram of optical amplifier 10' equipped with PM& SSD unit 110. Only the blocks pertaining to one direction oftransmission, namely from transmitter of terminal 60 to receiver ofterminal 70 are illustrated in FIG. 2 for simplification.

As indicated above, fractions of the optical incoming and outgoingsignals 2 and 3 are tapped at an optical amplifier and the transmissionsystem parameters are measured for both tapped portions of the opticalsignal.

Input optical tap 31 removes a 3-5% input fraction 11 from the incomingoptical signal 2 and output optical tap 32 removes a 3-5% outputfraction 14 from the outgoing optical signal 3. For an amplifier A_(i),the input fraction comprises an input dither signal D_(Ii) and theoutput fraction comprises an output dither signal D_(Oi). In case ofnormal functioning, the output dither will include a local dither D_(ij)which is added to the input dither D_(Ii). If the local dither isdetected in the input fraction, this is interpreted by the PM & SSD unitof this invention as a reflection.

Each of the input and output fractions is converted to an electricalsignal by a respective opto-electronic converter 33 and 34. Eachconverter 33 or 34 generally comprises a PIN photodiode and atrans-impedance amplifier (not shown). The PIN diode converts theincident light into an electrical current which is amplified and bandlimited between 0 and 1 kHz by the trans-impedance amplifiers. Theanalog signals obtained at the output of opto-electronic converters 33and 34 are a measure of the incoming optical signal 2 and outgoingoptical signal 3. They are respectively converted to digital signals bytwo slow analog to digital (A/D) converters 37 and 38 thatsimultaneously sample the levels of the analog signals at their inputsand pass the resulting data signals to an on-board control unit 7 toprocess. The control unit 7 processes these digital signals to producecontrol signals 40 and 41.

The first control signal 40 is obtained in control unit 7 by comparingthe estimated total power of the tapped optical incoming and outgoingsignals. Control signal 40 is converted to an analog power controlsignal 42 with digital-to-analog converter 43. Power control signal 42is applied to a first input of summing circuit 44 for setting theoperating point of the laser. The local dither D_(ij) (D_(1j) in theembodiment illustrated in FIG. 2) is applied to a second input of thesumming circuit 44.

The amplitude of the local dither is precisely controlled with signal 41which determines the dither modulation depth. The amplitude chosen is atrade-off between improved accuracy obtained with larger amplitudes andless transmission system impairment obtained with smaller amplitudes.Digital to analog converter 46 receives the second control signal 41 andconverts signal 41 into a modulation level signal 47 which is applied tochopper 48. Chopper 48 also receives a local code C_(ij) (C_(1j) in theembodiment illustrated in FIG. 2) supplied by the SSD unit 8 on line 65and accordingly modulates the analog control signal 47 with the localcode. The modulated signal 45 is applied to a band pass filter 49 whichlimits the band of the modulation to produce the local dither D_(ij)(D_(1j) in FIG. 2). The low frequency locally generated dither is mixedin unit 44 with power control signal 42 and the resulting signal 55 isapplied to laser source 9.

As indicated above, the low frequency modulation D_(ij) generated byeach amplifier A_(i) should be always uncorrelated to the dithers fromany other amplifier, data source, control circuit, or other dither. Thedither D_(ij) is obtained using a local code C_(ij) generated in theself-stimulation signal detection unit 8 so as to be unique to thetransmission system as indicated above.

Amplifiers 56 and 57 are provided for the AC component of the voltage ofthe incoming and outgoing signals. They are followed by the analog todigital converters 60 and 61 which convert the analog output of theseamplifiers to digital signals 62 and 63 that are transmitted to SSD unit8.

The first and second digital signals 62 and 63 are used for adjustingthe amplitude modulation of the local dither D_(ij), for selecting thelocal code C_(ij), and for diagnosis and isolation of faults.

Whenever the local dither is detected in the received dither, SSD 8alarms control unit 7 by issuing a fault signal 58, and the control unittriggers alarm block 59. The control unit also monitors the operation ofthe SSD 8. Data measured by the control unit 7 as well as the faultsignals may be transmitted to a remote monitoring unit at one of theoptical terminals and control signals may be received via remotemonitoring bus 54.

FIG. 3 shows a block diagram of the SSD unit 8 of amplifier 10',according to one embodiment of this invention. In FIG. 3, C_(1j) is thelocal code, and C_(j) designates any of the codes available for thetransmission system.

At each amplifier, a code generator 71 generates codes C_(j), which areused for detecting the codes present in the input and output dithersignals D_(Ii) and D_(Oi), or, more precisely, in the first and seconddigital signals, respectively.

Choosing local code C_(ij) is an important decision, having in view theconstrains imposed to this signal. The code must not impair thetransmission system, should be out of band for easy detection of theinformation on the link, and the neighboring amplifiers codes should notinterfere with each-other. Another requirement for this signal is to beselected such as to allow for an easy detection. This implies that thepower of the code be constant and precisely determined so as to easilydistinguish it over the noise.

As indicated above, codes that do not interfere with the codestransmitted by the other amplifiers in the link may be codes which arephase independent orthogonal to each other. Phase independentorthogonality implies that the components of the frequency spectrum of acode occur at different frequencies than those of the other code. Forexample, if the circuit includes four optical amplifiers 10, 20, 30 and40 as illustrated in FIG. 1, the initial codes C_(j) chosen are 50% dutycycle square waves of the following periods: T_(C1) =12 ms, T_(C2) =16ms, T_(C3) =18 ms, T_(C4) =24 ms and T_(C5) =32 ms. They will bemeasured in a master period T_(S) of 288 ms in which all five codes havean integer number of periods.

As well, a random number unique to the amplifier may be used as thelocal code.

The SSD unit 8 comprises code detector 66 for attempting to detect thelocal code in the first and second digital signals 62 and 63, and anoise estimator 69 for setting a noise threshold 68 for decisionsregarding the presence of the codes in signals 62 and 63.

The local code detector 66 detects the measured code C_(1j) in thesecond (output) signal 63 and applies this signal to the control unit 7as a code present signal 64. Signal 64 is compared to the estimatedtotal power of the tapped outgoing optical signal to produce themodulation level signal 41. The measured modulation depth of the codeC_(ij) measured in the digital signal 63 is also compared with the knownmodulation depth of the local code to estimate the signal and noisecomponents of the optical outgoing signal.

The measured code C_(1j) is detected using straight forward correlationdetection. The decision threshold signal 68 generated with noiseestimator block 69 is used for separating the detected code from thenoise.

More importantly, the local code detector 66 attempts to detect thelocal code C_(1j) in the input signal 62, while SSD unit 8 transmitsthis code. If the local code is detected in the first signal 62, this isperceived as a reflection and the control unit and remote monitors arealerted of this faulty situation by alarm block 59.

SSD unit 8 also comprises a code selection unit 72 which is used forselecting the local code for amplifier 10' to transmit from theplurality of codes available for the respective transmission system.Code selection unit 72 comprises a codes detector 67, a conflictdetector 74 and a priority select block 75.

The codes detector 67 measures the codes present in the first and secondsignals 62 and 63, using the codes signal 76, and produces a codespresent signal 66. Detector 67 is a bank of filters matched to eachorthogonal code C_(j). The decision threshold signal 68 is used by thecode detector 67 for pulling the codes out of the noise.

Conflict detection unit 74 is used to determine if the local code C_(ij)transmitted by amplifier A_(i) conflicts with any other code transmittedby the other amplifiers in the link. Conflict detection unit 74 comparesthe local code C_(ij) with the codes present signal 73 and changes thelocal code if a code conflict is detected, using code change signal 77.Priority select block 75 receives the codes signal 76 from the generator71 and instructs the generator to select the local code based on apriority scheme. Preferably, the amplifier closest to terminal 70 willtransmit the highest priority code in the link and the amplifier closestto the terminal 60 will transmit the lowest priority code in the link.

The SSD unit of this invention operates in two distinct modes, namelythe code selection mode and reflection detection mode. The codeselection mode is initiated when the amplifier is powered on. This modeinvolves detecting the codes present in signals 62 and 63, initialselection and transmission of a local code C_(ij), code resolution whena code conflict exists and finally, selecting a local code for thereflection detection mode. After the local code has been selected, thereflection detection mode is initiated, and the presence of reflectionsis detected. Concurrently, a periodical check to determine whether thelocal code is uncorrelated to any other code present in the link is alsoeffected at random moments.

Code Selection Mode

The code selection mode will be described in connection with FIGS. 3 and4. FIG. 4 illustrates a flowchart of the code selection method when theoutput of each amplifier in the link is dithered with a different code.

An initial selection takes place at each amplifier when it is poweredon, followed by a conflict detection for verifying if it is the onlyamplifier transmitting the code initially selected.

As indicated above, the codes detector 67 detects the codes in the firstand second digital signals 62 and 63 while code generator 71 does nottransmit any code. Generator 71 selects a local code C_(1j) based onthis measurement.

Initial Code Selection

Firstly, the generator 71 selects a code C_(1j) for the amplifier 10,out of codes C_(j) that may be generated by it. This operation isexecuted whenever the amplifier is powered on or reset.

First, the amplifier A_(i) executes its standard initialization in step80 and the SSD unit 8 is reset so that no code is output on terminal 65,as indicated by C_(ij) =0. Code detector unit 67 receives all codesC_(j) on line 75 and the input and output signals 62 and 63 and detectsthe codes that are present in these signals, in step 81. This involvescorrelating for each M orthogonal codes. One sampling period of data iscollected to this end, with no dither being transmitted. Correlationwill be done on this sample with all codes.

Then, in step 82, generator 71 selects a code C_(ij) for amplifier A_(i)to transmit based on priority signal 78, so that C_(ij) does notconflict with any of the codes transmitted by the other amplifiers inthe link.

Code Conflict Detection

After the initial code selection is complete, on-line detection of codeconflicts is performed in conflict detector block 74, so that anyconflicts are resolved by selecting a new local code for the amplifierunder consideration. It is possible that a code conflict may exist, evenafter the initial selection has been made based on the codes transmittedby the other amplifiers. This could occur due to several differentmechanisms, but should be generally infrequent. The presence of thelocal dither may cause code conflicts with other amplifiers in the link,resulting in code shuffling on some or all of the amplifiers in thelink. To accommodate this shuffling, each SSD unit will continue thecode selection until six trials have been elapsed in which it was notrequired to change the code.

If a conflict exists, conflict detector 74 receives the local codesignal 65 and the codes present signal 66. The conflict detector unit 74will detect the local code C_(j) in the signal 73 and will instruct thegenerator 71 to change the local code through code change signal 77.

The SSD unit checks if the code initially selected has been changed instep 83, and if not (C_(j) =C_(j-1)), a counter COUNT is incremented forcounting up to six trials in step 84. When six trials were performedduring which the code selection unit was not required to change thecode, as shown in step 85, the amplifier A_(i) will transmits its codeC_(ij) (step 86) for reflection detection (step 87).

If in step 83 it is detected that code C_(ij) was changed, the counteris reset in step 88. This implies that after a code change, six trialsshould be again run, so as to account for code shuffling. The dither ismeasured in step 89 to detect all codes present, in step 90 the codedetector 69 checks if the local code C_(ij) is present at its inputs andif not, same code C_(ij) is maintained and six trials are performed asdiscussed above. If C_(ij) is detected in the codes present signal 66, anew code C_(i),j+1 is selected by priority select block 75 in step 91,the dither is again measured and the steps shown in FIG. 4 are executedas described above.

There is a potential for a problem if synchronization between amplifiersexists. For example, if two or more amplifiers are transmitting the samecode and they discontinue transmission at the same time, an eventualcode conflict will never be detected and an incorrect code selectionwould be made. To prevent this potential synchronization problem fromoccurring, the dither is measured in step 89 for a random lengthinterval of between 12 and 44 master sample periods. The probability ofsynchronization existing through six iterations is: ##EQU1##

This probability is acceptably low. If synchronization exists after sixiterations, and a code conflict exists, the conflict detector willdetect it and the situation will be rectified.

Following this, SSD will begin detecting if any reflected signal ispresent at its inputs, along with checking randomly for code conflict.

Selection Of Codes Priority

The generator 71 selects the code C_(ij) on a basis of a priorityscheme, which will be explained in the following in connection withFIGS. 1 and 5. Amplifier 40 closest to receiver/transmitter 70 willtransmit the highest priority code on the link, while amplifier 10closest to transmitter/receiver 60 will receive the lowest priority codein the link. Once the initial code selection is complete, the amplifierwill begin to transmit its code. This process will repeat at randomintervals, allowing for the fact that some code shuffling betweenamplifiers may occur initially.

The priority of the codes is implemented preferably in a circularfashion to minimize the amount of code shuffling among the amplifiers inthe link. Where, for example, the three amplifiers closest to thetransmitter/receiver 60, 10, 20 and 30 have selected codes correctly,the fourth amplifier 40, closest to the receiver/transmitter 70 was thelast amplifier in the link powered on, and hence the last amplifiertrying to perform code selection. Table 1 illustrate this case.

                  TABLE 1                                                         ______________________________________                                        Amplifier A.sub.1 A.sub.2  (20)                                                                            A.sub.3  (30)                                                                       A.sub.4  (40)                              ______________________________________                                        Code k    2       1          0                                                (k = j - 1)                                                                   Time T.sub.1                                                                            2       1          0     0                                          Time T.sub.2                                                                            2       1          1     0                                          Time T.sub.3                                                                            2       2          1     0                                          Time T.sub.4                                                                            2       2          1     0                                          Time T.sub.5                                                                            3       2          1     0                                                    OR                                                                  Time T.sub.1                                                                            2       1          0     4                                          ______________________________________                                    

Normally priority "0" (j=1) is the highest priority and "4" (j=5) is thelowest. If this was adhered to rigidly and a circular priority schemewas not used, amplifiers 10, 20 and 30 would all be required to changecodes. Amplifier 40 would measure code C₁ with priority "0" in T₁,amplifier 20 would detect its conflict with amplifier 10 in T₂ anddecrement its code. Then amplifier 30 would detect its conflict with 20in T₃, and so on. However, with circular priorities, code C₅ ofamplifier 40 becomes higher priority than code C₁ of amplifier 30 and noshuffling is required. Thus, the fifth code can be used for expeditingthe initial code shuffling.

A pseudocode description of this method is illustrated in FIG. 5. FIG. 5illustrates how a SSD unit selects the code to be transmitted accordingto the "low priority" or "high priority" routine.

In step 95 amplifier 30, for example, is initialized and begins codeselection. The code detection unit 72 of the amplifier checks if anycode C_(ij) is received at input 19 and output 22 in step 96. If no codeis detected, this means that amplifier 30 is the first amplifier whichis ready to select its code. Code C₁ with the priority "0" is selectedin step 97. If in step 98 amplifier 30 detects a code in the seconddigital signal (output), this means that downstream amplifier 40 hasbeen initialized before amplifier 30, and already transmits its code.The code transmitted by unit 40 should have a higher priority than thatof unit 30, and a "high priority" routine is executed in step 100. If instep 98 amplifier 30 detects a code in the first digital signal (input),this means that upstream amplifiers 10 or/and 20 have been initializedbefore amplifier 30, and already transmit their codes. The codetransmitted by units 10 and 20 should have a lower priority than that ofunit 30, and a "low priority" routine is executed in step 99.

The following example illustrates how priority is attributed to the lasttwo codes which are not yet transmitted by any amplifier.

    ______________________________________                                        Code           Present Priority                                               ______________________________________                                        1              True                                                           2              True                                                           3              False   High                                                   4              False   Low                                                    5              True                                                           ______________________________________                                    

The "high priority" routine selects the highest priority code using acircular scheme. This code is always the first free code, after thefirst unused code, searching from 0 to 4. If a code C₄ is reached in thesearch, the routine will wrap around and continue searching at code 0.The following example illustrates this case.

    ______________________________________                                        Code           Present Priority                                               ______________________________________                                        1              False   High                                                   2              False                                                          3              False                                                          4              False   Low                                                    5              True                                                           ______________________________________                                    

The "low priority" routine selects the highest priority code in asimilar fashion. This code is always the first free code, after thefirst unused code, searching in reverse from 4 to 0. If a code C₀ isreached in the reverse search, the routine will wrap around and continuesearching at code C₄. The following example illustrates this case.

    ______________________________________                                        Code           Present Priority                                               ______________________________________                                        1              True                                                           2              False   High                                                   3              False                                                          4              False                                                          5              False   Low                                                    ______________________________________                                    

As discussed above, there is a potential for a problem ifsynchronization between amplifiers exists. To prevent this potentialproblem, generator 71 disables the local code output by block 71 andinitiates detection of codes present. The detection of codes on inputs62 and 63 is performed for between 12 and 44 master sample periods.

Reflection Detection Mode

This involves monitoring the presence of reflections as well asperiodically checking whether the local code is uncorrelated to anyother code present in the transmission system.

It is important that a fault be declared quickly in the detection mode.The reflection detection mode has a faster response time than the changeof a local code in the code selection mode, for more accuratemeasurement of reflections. The minimum reflection detectable can bepreset by modifying the noise threshold signal.

Conflict detection unit 74 checks for eventual conflicts at randomintervals. The code generator 71 will discontinue transmission of thecode C_(ij) for a short period and the unit will attempt to detect thesame code in the first and second digital 62 and 63. As in the case ofthe initial selection of the local code, if the code is present when thecode generator is not transmitting, a code conflict exists. To resolvethis conflict, conflict detection unit instructs generator 71 on line 77to change the local code and so on, as explained above.

FIG. 6 shows the block diagram for another embodiment of SSD unit 8. Thelocal code C_(i) is generated in the random number generator 78 usingfor example the last significant digits from the pump accumulator. Thelocal code is then applied to laser source 9 using chopper 48, band passfilter 49, and summing circuit 44, as in the case of orthogonal codesC_(ij). The light emitted by the laser is dithered with this lowfrequency signal that is an independent function of the respectiveamplifier.

This dither D_(i) is applied at random moments and allows detection of areflection when the information is present or not. The SSD unit 8comprises code detector block 66 and noise estimator block 69. Fordetecting eventual reflections, the first digital signal 62 and thesecond digital signal 63 are applied to the noise estimator 69 and codedetector 66. The local code C_(i) is first pulled out from the noise bycorrelating over 256 samples using noise threshold signal 68. The amountof the code is then used to compute the amount of reflection present andthe amount of input actually present in the output signal. If C_(i) isdetected in the D_(Ii), the fault signaling unit 73 alerts the controlunit 7, which triggers alarm 59.

FIG. 7 shows how the local code C_(i) may be generated with the SSD unit8 of FIG. 6.

In block 101 the amplifier module is initialized and in block 102 the 16least significant bits (LSB) PA of the 32 bit pump accumulator of thepump power controller are read in field P. This is a fairly randomnumber as it is strongly influenced by the noise history and thecharacteristics of the particular pump of that amplifier. This number isthe seed to start the random number generator which will then run untilthe amplifier is reset/powered down.

The number in field P is shifted during an interval T, which is, forexample, 256 ms, as illustrated at 104. After period T expires, asdetected in block 103, the 16 least significant bits (LSB) PA of the 32bit pump accumulator are read again in field R in step 105. The number Rdepends on the current conditions at the pump accumulator, as thecurrent temperature, current level of noise, etc. In step 106, thenumber in field R is XORed with the number in field P. The output bitsof the XOR circuit are then encoded in block 107 to obtain code C_(i).For example, Manchester encoding may be used, where there is atransition at the middle of each bit period; a low to high transitionrepresents a 1, and a high to low transition represents a 0. This typeof encoding is preferable in that the band obtained with the Manchestercoding is broad and it does not have any DC component.

This is the code C_(i) for the next 256 ms, which is unique to the pumpaccumulator of the respective amplifier and therefore unique to theamplifier A_(i). As in the case of the previous embodiment described, amodulation of the output signal and noise is used as probe signal,rather than introducing a new optical source to generate a probe. Thus,the device and method of this invention operates with or without a datasignal being present.

While the invention has been described with reference to particularexample embodiments, further modifications and improvements which willoccur to those skilled in the art, may be made within the purview of theappended claims, without departing from the scope of the invention inits broader aspect.

What is claimed is:
 1. An apparatus for detecting a fault at an opticalamplifier module of a transmission system, comprising:means formodulating a laser of said optical amplifier module with a local dither;means for measuring an outgoing optical signal of said optical amplifiermodule to obtain a transmitted dither; means for measuring an incomingoptical signal of said optical amplifier module for detecting a receiveddither; and means for processing said received dither and saidtransmitted dither for determining the ratio of the energy in saidreceived dither to the energy in said transmitted dither, indicating ifsaid local dither is present in said incoming optical signal.
 2. In anoptical transmission system with a plurality (N) of optical amplifiers,an apparatus for detecting a fault at each amplifier A_(i) (iε 1,N!)comprising:means for modulating a laser of said optical amplifier A_(i)with a local dither D_(ij) unique to the transmission system; means formeasuring an outgoing optical signal to provide a respective transmitteddither D_(Oi) ; means for measuring a respective incoming optical signalfor detecting a respective received dither D_(li) ; and means forprocessing said respective received dither D_(lj) and said respectivetransmitted dither D_(Oi) for determining the ratio of the energy insaid respective received dither to the energy in said respectivetransmitted dither to detect if any local dither D_(fj) is present insaid received dither D_(lj), where f designates said amplifier A_(i) andany other amplifier connected in the transmission system downstream fromsaid amplifier A_(i).
 3. A method for monitoring the performance of anoptical amplifier module of a transmission system comprising the stepsof:generating at said optical amplifier module a local dither;modulating the bias current of an optical source of said opticalamplifier module with said local dither; measuring an outgoing opticalsignal of said amplifier module to provide a transmitted dither;measuring an incoming optical signal of said amplifier module fordetecting a received dither; and determining the ratio of energy in saidreceived dither to the energy in said transmitted dither fortracking-down said local dither in said incoming optical signal.
 4. Amethod as claimed in claim 3, wherein said optical source is a laser forthe data communication.
 5. A method as claimed in claim 3, wherein saidoptical source is a laser for the overhead communication.
 6. A method asclaimed in claim 3, wherein said local dither is a pseudorandom codesequence unique to said optical transmission system.
 7. A method asclaimed in claim 3, wherein said local dither is a pseudorandom codesequence which is uncorrelated to any other signal in said opticaltransmission system.
 8. A method as claimed in claim 3, wherein saidstep of measuring said incoming optical signal comprises:diverting afraction of said incoming signal; estimating the energy of noise in saidincoming optical signal and accordingly providing a noise threshold;comparing the amplitude of said received dither to said noise threshold;and estimating the energy of said received dither in said fraction ofsaid incoming signal.
 9. A method as claimed in claim 3, wherein saidstep of measuring said outgoing optical signal comprises:estimating ameasured transmitted dither and a measured total power of said measuredoutgoing signal; estimating a signal and a noise component of saidmeasured outgoing signal by comparing the amplitude of said measuredtransmitted dither to said measured total power; producing a firstcontrol signal dependent of said measured power; and producing a secondcontrol signal for controlling the amplitude of said transmitted dither.10. A method as claimed in claim 9, wherein said step of generatingcomprises the sub-steps of:generating a pseudorandom code sequenceunique to said optical system with a code generator; modulating saidfirst control signal with said code for obtaining a dither signal; andfiltering said dither signal to obtain a filtered dither signal.
 11. Amethod as claimed in claim 9, wherein said pseudorandom code sequence isobtained by combining the output of a pseudorandom number generator witha symbol sequence that is a function of said amplifier.
 12. A method formonitoring the performance of a WDM optical transmission systemcomprising the steps of:at each optical amplifier A_(i) (iε 1, N!) ofsaid transmission system,generating a local dither D_(ij) (iε 1, N!, (ε1, M!) of a known energy, for identification of said optical amplifierand of a transmission channel; modulating the bias current of a laser ofsaid transmission channel with said local dither to mix an outgoingsignal with said local dither; measuring an incoming optical signal fordetecting a received dither D_(li) ; measuring said outgoing opticalsignal for detecting a transmitted dither D_(Oj) ; calculating the ratioof energy in said received local dither to the energy in saidtransmitted dither; and determining if a local dither D_(fj) is presentin said incoming optical signal, where f designates said amplifier A_(i)and any amplifier connected in said transmission system downstream fromsaid amplifier A_(i).
 13. A method as claimed in claim 12, wherein saidlocal dither has statistics that are substantially independent of allother dithers present in said transmission system.
 14. A method asclaimed in claim 12, further comprising the step of selecting a localdither D_(ij) out of an given bank of M dither signals D_(j) (jε 1, M!)such that said local dither D_(ij) is phase orthogonal with each otherlocal dither D_(km) generated by each other amplifier A_(k).
 15. Amethod as claimed in claim 14, wherein said step of selectingcomprises:initializing said amplifier A_(i) ; measuring an outgoingoptical signal for detecting a transmitted dither D_(Oi) of saidamplifier A_(i) ; processing said transmitted and said received ditherto determine dithers D_(j) absent in both said signals; and selectingsaid local dither D_(ij) out of said absent dithers D_(j).
 16. A methodas claimed in claim 15, wherein said steps of measuring said incomingoptical signal comprises:tapping a fraction of said incoming opticalsignal; and estimating said received dither D_(Ii) in said tappedincoming fraction by correlating said received dither with each of saiddithers D_(j) to obtain a plurality of input picks.
 17. A method asclaimed in claim 16, wherein said steps of measuring said outgoingoptical signal comprises:tapping a fraction of said outgoing signal; andestimating said transmitted dither D_(Oi) in said tapped outgoingfraction by correlating said transmitted dither with each of saiddithers D_(j) to obtain a plurality of output picks.
 18. A method asclaimed in claim 16, wherein said step of calculatingcomprises:estimating the energy of noise in said incoming and outgoingsignals and calculating a noise threshold; comparing the amplitude ofeach input pick to said noise threshold to determine if any of saidlocal dithers D_(ij) is present in said incoming signal; comparing theamplitude of each output pick to said noise threshold to determine ifany of said local dithers D_(ij) is present in said outgoing signal; anddetermining all dithers D_(ij) out of said dithers D_(j) which areabsent in said incoming and outgoing signals.
 19. A method as claimed inclaim 12, further comprising the steps of:determining the power of saidincoming signal and the power of all said local dithers D_(fj) detectedin said incoming optical signal; and calculating the actual power ofsaid incoming signal by subtracting the power of all said local dithersD_(fj) from the power of said incoming signal.